Membrane switches are typically built as normally open switches. However, several normally closed switches do exist in the field of art. For example, U.S. Pat. No. 4,771,139 issued to DeSmet discloses an improved keyboard having a flexible metal cover, normally closed switches, and multiple throw switches. The normally closed switch described in DeSmet includes a non-conductive pellet which transmits the actuating force on a key or switch through a substrate on which the switch is mounted. The substrate includes a hole through which the pellet can extend. The pellet provides a means for communicating the actuating force of a key site through the substrate to a movable electrical contact which is normally closed. When the switch is pressed, it extends through an opening in the substrate and pushes the electrical contacts out of communication thereby breaking the electrical circuit of the switch. The switch configuration includes a leaf spring having first and second ends which are in communication with contacts that are located on the substrate. At least one end of the spring is fixed or soldered to its associated contact.
Further, U.S. Pat. No. 4,618,754 issued to Gross describes a switch with a pivotable rocker that is arranged in a normally closed configuration. The switch has a normally closed set of electrical contacts and a downward force applied to the switch pivots an elongated, flat rocker to open the normally closed contacts. Pivoting of the rocker is yieldably resisted by an overlaying resilient membrane which forcibly returns the rocker to its original position when the force is removed. However, the rocker in the normally closed switch in Gross is located immediately above an upper circuit board having a first set of spaced electrical contacts fixed on its upper surface and a lower circuit board having a second set of spaced electrical contacts fixed on its upper surface. The upper surface board is positioned over the lower circuit board and an aperture in the upper circuit board is aligned with the second set of electrical contacts. The rocker comprises first and second ends having electrical contacts located on the bottom of the rocker at each end. The rocker rests on the upper circuit board with its first contact end touching and electrically shorting together the first set of circuit board contacts while the rocker's second contact end is located slightly above the second set of circuit board contacts. Accordingly, the first set of circuit board contacts provides a normally closed switch configuration while the second set of circuit board contacts simultaneously provides a normally open switch configuration.
A second embodiment of a momentary membrane switch having both a normally open set of electrical contacts and a normally closed set of electrical contacts is also disclosed in the Gross patent. This second embodiment comprises a disk-shaped switch element and a circuit board having first and second sets of circuit board contacts. The disk-shaped switch element has a first switch contact located on a circular ridge projecting downwardly and outwardly from the periphery of the disk-shaped element and a second switch contact located on a shallow projection in the center of the switch element's underside. The switch is assembled such that the first switch contact touches and shorts together the first set of circuit board contacts and the second switch contact is aligned with, and spaced slightly above, the second set of circuit board contacts. Accordingly, the first switch contact and first set of circuit board contacts form a membrane switch which is in a normally closed position and the second switch contact and second set of circuit board contacts form a membrane switch which is in a normally open position. However, upon applying a downward force to the center of the disk-shaped switch element, the second switch contact touches and shorts together the second set of circuit board contacts while the circular ridge which contains the first switch contact on the switch element's underside deforms upwardly and away from the circuit board thereby lifting the first switch contact away from the first set of circuit board contacts to open that switch. Once again, as described with reference to the first switch embodiment disclosed in Gross, this switch includes a normally closed set of electrical contacts and a second set of contacts that are simultaneously in an open configuration.
As can be seen from the above descriptions of normally closed membrane switches that currently exist in the field of art, these membrane switches require additional elements and space requirements compared to switches having a normally open configuration in order to perform their function. Further, in that there are situations and circumstances in which a normally closed switch configuration is desired, there is a need for a simply constructed yet durable normally closed membrane switch which has all of the attributes of a membrane switch having a normally open configuration.